I saw a reference that uranium reacts strongly, and is never found pure in nature. In many instances the melt which has migrated from the initial magma chamber will undergo further segregation. The bigger c1 is, the older the rock is. Snelling discusses numerous false ages in the U-Pb system where isochrons are also used. What about the radiometric assumption of constant decay rate? Radiogenic Isotope Geology.
This can happen by mixing scenarios as indicated above.
Accuracy of Fossils and Dating Methods
So all the mechanisms mentioned earlier are capable of producing isochrons with ages that are too old, or that decrease rapidly with time. I am just reporting what HE said! I read that zircons absorb uranium, but not much lead. These will be definite factors that will change relative concentrations of parent and daughter isotopes in some way, and call into question the reliability of radiometric dating. Both disciplines work together hand in hand, however, to the point where they share the same system of naming rock layers and the time spans utilized to classify layers within a stratum. Let me make some general comments about isochrons.